Brent was born in Main Street, Ballinasloe, County Galway, on March 15, 1904,[1][2][3] to John J. and Mary (née McGuinness) Nolan. His father was a shopkeeper and his mother was a native of Clonfad, Moore, County Roscommon.[4] In September 1915,[4] he moved with his younger sister Kathleen to New York City. There, they joined their mother, who was living in the US after her separation from her husband.

He fled Ireland with a bounty set on his head by the British government, although he later claimed only to have been a courier for guerrilla leader and tactician Michael Collins. According to Ballinasloe Life (volume 2, issue 4, Oct/Nov 2012),[2] the Irish War of Independence careers of three different men named George Nolan (Brent and two others; one from County Dublin and the other from County Offaly) were apparently conflated, which may explain some of the discrepancies regarding Brent's year of birth, life, and activities during the 1919 to 1922 period.[5][7][8]

Brent traveled from England to Canada and returned to the United States in August 1921.[9][10]

He decided to become a professional actor. He made his Broadway debut in director Guthrie McClintic’s The Dover Road. He did numerous plays throughout the 1920s, including running several of his own stock companies. He appeared in productions of Abie's Irish Rose (on tour for two years), Stella Dallas, Up in Mable's Room, Elmer the Great, Seventh Heaven, White Cargo and Lilac Time. He acted in stock companies in Colorado, Rhode Island, Florida, and Massachusetts. In 1930, he appeared on Broadway in Love, Honor, and Betray, alongside Clark Gable.[5][11][12]

In 1942, Brent, an accomplished pilot who had tried and, because of age, failed to enlist in the armed services, temporarily retired from films to teach flying as a civilian flight instructor with the Civilian Pilot Training Program and later became a pilot in the US Coast Guard[21] for the duration of the war.[22]

His final film for Warner Bros. was My Reputation with Barbara Stanwyck as a widow that was filmed from November 1943 to January 1944, and with the exception of military audiences, was not released until 1946.[21] Brent acted on radio during this period.[23]

His final marriage to Janet Michaels, a former model and dress designer, lasted 27 years until her death in 1974. They had two children together: a daughter, Suzanne (born August 3, 1950), and a son, Barry (born November 26, 1954).[30]

Brent also had affairs with actresses Greta Garbo and Bette Davis, the latter a frequent Warner Bros. co-star.